In supercapacitors and ultracapacitors, surveyed, for example in Schindall, The Charge of the Ultracapacitors, IEEE Spectrum, vol. 44, pp. 42-46 (2007), nanomaterials have been used to increase charge density, however the enhancement of dielectric strength with decreasing gap size between electrodes separated by a solid dielectric has never been addressed.
Insofar as the volumetric energy density of energy stored in a capacitor is Ud=∈0∈E2, where ∈0 is the vacuum permittivity, £ is the dielectric constant characterizing the dielectric, and E is the electric field in the dielectric, stored energy is optimized by maximizing E. For a parallel plate geometry, the electric field is the applied voltage V divided by the capacitor spacing d, so stored energy density scales as (V/d)2, and correspondingly for other geometries.